1. Field of the Invention
The present invention relates generally to an optical time domain reflectometer (hereinafter, referred to as "OTDR"), and more particularly to an OTDR using an optical element with three control modes of oscillation, attenuation and amplification.
More specifically, this invention relates to an OTDR for measuring transmission characteristics of an optical fiber and, for example, identifying the location of a fault in an optical fiber transmission path. In particular, in this invention, when a test light pulse is generated, an optical element is used as a light source. When a pulse refected from an optical fiber is received, the optical element is used as a light amplifier or a light attenuator. According to the OTDR of this invention, saturation of a light receiver due to Fresnel reflection can be prevented, and Rayleigh scattering light can be measured.
2. Description of the Related Art
In a typical OTDR, an acoustic/optical element (A/O element) with small loss is provided between an light source, an optical fiber, and a receiver, thereby changing the light path. This A/O element, however, is expensive, and optical alignment between a transmission path and a reception path is difficult.
U.S. Pat. No. 4,070,118 discloses an OTDR wherein an optical element (e.g. laser) is employed as an oscillator or an amplifier, without providing an A/O element. This OTDR will now be described with reference to FIG. 1. When a laser 61 is supplied with a current pulse, exceeding an oscillation limit value, it generates a test pulse. The test pulse 65 is applied to a light transmission body 62. On the other hand, a light pulse 66 is generated from the laser 61 and is applied to a light receiver 64 thereby marking a zero point on a time scale. After the test pulse is generated, the laser 61 lowers its operation current slightly below the oscillation limit value, in order to perform an amplification function. A reflected pulse 67 reflected from a fault point of the light transmission body 62 is optically amplified by the laser 61 when it passes through the laser 61. The amplified pulse 67 is applied to the light receiver 64. The light pulses 66 and 67 input to the light receiver 64 are converted to electric signals, and the electric signals reach outputs A. The signals output from the light receiver 64 are observed by an oscilloscope (not shown in FIG. 1) or are recorded on a recording device.
The conventional OTDR shown in FIG. 1 has the problem, as is explained by referring to FIG. 2.
Recently, there has been a considerable demand for an OTDR capable of measuring Fresnel reflection light and Rayleigh scattering light, that is, for detecting a fault point of an optical fiber and transmission characteristics. The conventional OTDR shown in FIG. 1, however, is designed mainly to measure Fresnel reflection light, and is not designed to measure Rayleigh scattering light. Thus, if this conventional OTDR is employed to measure both Fresnel reflection light and Rayleigh scattering light, the Rayleigh scattering light (indicated by a broken line D in FIG. 2) is masked (as shown by a solid line C in FIG. 2) and cannot be measured.
In general, in this type of apparatus, the laser 61 and the light transmission body 62 are coupled by a connector (not shown). The reflected light passing through the laser 61 includes a Fresnel's reflection light component (denoted by B in FIG. 2), a Fresnel's reflection light component (denoted by C in FIG. 2) and a Rayleigh scattering light component occurring at the fault point 63. In particular, the Fresnel's reflection light has a high power and is amplified through the laser 61, thus saturating the light receiver 64. Once the light receiver 64 is saturated, it takes a considerable amount of time for the light receiver 64 to be restored to the normal state. Consequently, the OTDR shown in FIG. 1 fails to observe a peculiar point existing just after the connector or the fault point 63 (e.g. component D in FIG. 2, or a slight loss due to a welded point for connection of an extended light transmission body).